In this study we shall attempt to define the immunologic events that occur during the latent period of mammary tumor development. We shall study the role of T and B cells, in the control of tumorigenesis both as independent entities and under conditions where there is interaction. CMI (T cell function) will be studied in vivo by measuring GVH reactivity and assesssing the capacity to produce specific killer cells and in vitro be measuring responsiveness to selected mitogens. B cell function and humoral immunity will be evaluated by both response to B cell mitogens and the development of blocking antibodies. These studies will be done in both thymectomized and sham operated animals and in animals with thymus implants, in order to assess the effects of thymus integrity on the process of tumorigenesis. We shall study immune functions in animals with chemically induced (benzpyrene) tumors and under conditions of enhancement (Sarcoma I -C57BL) in an effort to ascertain whether or not surveillance is operative in all cases and if such a response is beneficial to the host. Finally by selected matings and foster nursing with three strains of mice we shall study the relationship between immune response genes and reisitance to mammary tumor development.